1. Field of the Invention (Technical Field)
The present invention relates to methods and apparatuses for common alignment of pixels from two or more different optical systems.
2. Description of Related Art
The spectral characteristics and signatures of natural and/or manmade objects vary. These differences may be employed in sensory instruments, where the discrimination of targets from background clutter is enhanced by the difference in spectral characteristics. In order to develop the algorithms needed to exploit these differences, a means is needed to create imagery in two or more spectral bands where each point of the field of view of two or more separate cameras align to a fraction of a pixel. Many systems have been design to generate imagery in two or more spectral bands, but none have successfully matched two separate optical systems to such accuracy. The field of view, focal length and distortion must be custom matched to meet this extremely difficult specification. Diffraction limited performance is also required from each camera optic.
Accordingly, critical alignment of each pixel of two optical systems operating in two or more spectral bands presents a difficult design challenge. Even extremely tight fabrication tolerances cause sufficient variation in the pixel alignment to defeat the purpose of simultaneous two color data collection and two-color algorithm development.
The present invention overcomes this difficulty by incorporating an afocal zoom optic located in front of one of the optical systems. The midwave and longwave optics preferably have substantially identical fields of view and substantially zero distortion so that each point in object space matches at the corresponding image point in the focal plane. Fabrication tolerances upset this delicate balance but adjusting the afocal zoom lens can readily make compensation for these variations.
Prior practices include the following. One approach is to use a single optic that is color corrected over the whole spectral band and a single detector sensitive to both spectral ranges. This approach, however, limits the types of detectors that can be used and the spectral ranges that can be imaged simultaneously.
A rotating filter wheel might be employed that alternates the image spectrum thereby achieving the pixel registration. However, the time delay between frames can lead to erroneous results if there is motion in the FOV. This approach requires a single focal plane array that therefore limits the spectral range that may be used to the sensitivity range of the detector.
Two separate lens systems have been used side by side to accomplish the same task as the invention. However, when the optics are arranged side by side the system suffers from parallax. Even when the apertures are combined with a dichroic the system is not pixel aligned because of dissimilar distortion characteristics between the two optical systems. Even when zoom optics are employed there are significant differences so that pixel match is not achieved.
Still another technique has been to employ four micro lenses directly in front of the detector array. Each lens is designed for a specific wavelength and has a spectral filter installed or deposited on the lens. The lenses are located in the cooler dewar and must be cooled along with the focal plane array. Each lens uses ¼ of the array. Since the lenses are so close together the parallax is insignificant, but the tolerances on the alignment and components are prohibitive. Since a single array is used, there are limits on the wavebands that can be covered simultaneously.
Also of note are U.S. Pat. No. 5,729,376, entitled “Catadioptric Multi-Functional Optical Assemby”, to Hall et al.; U.S. Pat. No. 5,477,395, entitled “Two Nested All-Reflective Afocal Telescopes Providing Four Fields of View”, to Cook; U.S. Pat. No. 5,078,502, entitled “Compact Afocal Reimaging and Image Derotation Device”, to Cook; U.S. Pat. No. 4,804,258, entitled “Four Mirror Afocal Wide Field of View Optical System”, to Kebo; and U.S. Pat. No. 4,691,999, entitled “Afocal Beam Expansion Apparatus and Method”, to Wheeler.
The present invention is simple and components may be fabricated to readily achievable tolerances. The data may be taken in all spectral bands simultaneously.